The present invention is directed towards proposing a process for preparing hydrogels based on a crosslinked polysaccharide, preferably hyaluronic acid, or a salt thereof.
Hyaluronic acid, which is naturally present in the skin, is known for its viscoelastic properties and its immense water-absorbing capacity. Its properties contribute largely to the elasticity of the skin.
It is precisely with regard to these properties that this compound has been exploited for more than 10 years in numerous applications falling within the medical and cosmetic fields, such as cosmetic surgery, dental surgery, articular viscosupplementation or ophthalmology.
Thus, hyaluronic acid is especially used for filling wrinkles and for attenuating, or even eliminating, local collapse of the structure of the dermis in the form of a wrinkle, generally via direct injection into the dermis, at the site of the wrinkle concerned.
In point of fact, hyaluronic acid is used essentially in the form of a crosslinked gel, given the increased resistance of this particular form to heat-mediated degradation, and thus to sterilization.
These crosslinked hyaluronic acid gels may be obtained via various preparation processes. In general, these processes require two main steps, the first consisting in hydrating the hyaluronic acid to convert it into an aqueous gel and the second directed towards crosslinking said aqueous gel in the presence of an agent that is capable of inducing its crosslinking (also known as a “crosslinking agent”).
As illustrations of these processes, mention may be made especially of those described in documents US 2006/0 105 022, WO 2006/056 204 or US 2007/0 036 745.
For obvious reasons, improving the mechanical properties of hydrogels, based on a crosslinked polysaccharide, this objective being liable to proceed via improving the processes for preparing these hydrogels, is an ongoing aim.
In this respect, FR 2 997 085 describes a process including the presence of an alkaline halide salt, especially of NaCl, during the implementation of the crosslinking reaction. This alkaline halide salt makes it possible to improve the rheo logical properties of the crosslinked gel.